judging that sps peice has come from your, as you said, so/so fish store, I would guess the BN is brown from nutrient issues. A little nitrate/phosphate can ruin the color in your sps polyps. i would guess thats why theirs are brown. Yours took a trip and is very small. probly lost some color during the trip.

Give yours some time. I would figure, in your tank, with your tank maintanance, yours will have nice green tips on the polyps soon.